Lubricating oil compositions, such as crankcase oils, contain various additives to improve their performance. Among the additives are dispersants which keep harmful particles suspended in the oil. These dispersants are often functionalized polymers wherein the polymer is a poly alpha-olefin such as polyisobutylene.
The polyisobutylene polymers (PIB) employed in most conventional dispersants are based on a hydrocarbon chain of a number average molecular weight (M.sub.n) of from about 900 to 2500. PI having a M.sub.n of less than about 300 gives rather poor performance results when employed in dispersants because the molecular weight is insufficient to keep the dispersant molecule fully solubilized in lubricating oils. On the other hand, high molecular weight PIB (M.sub.n &gt;3000) becomes so viscous that conventional industrial practices are incapable of handling this product in many operations. In addition, dispersants containing PIB result in significant engine wear in low phosphorous formulations or when used with dihydrocarbyl dithiophosphate DDP metal salts containing primary alcohol groups.
Since DDP metal salts with primary alcohol groups and low phosphorous oil formulations are very desirable in increasing fuel efficiency, it would be advantageous to use a dispersant that has good antiwear performance in low phosphorous oil formulations and in the presence of primary DDP metal salts. This is surprisingly accomplished in the present invention by using dispersants containing poly alpha-olefin polymer backbones with a molecular weight above 2500 (M.sub.n &gt;2500).